contacts-keeper.js

  sortLetters() {
    this.dataService.contacts[0].forEach(contact => {
      this.letters.push(contact.name.toUpperCase().slice(0, 1));
    });

    this.letters = this.letters.filter( (elem, index, self) => index === self.indexOf(elem));
  }

  sortContacts() {
    this.dataService.contacts[0] = this.dataService.contacts[0].sort((a, b) => a.name.localeCompare(b.name))
  }